Combined apparatus for rolling seamless tubes

ABSTRACT

The present invention relates to a plant  10  for rolling a seamless tube. The plant comprises a rotary piercer  20  which is designed to pierce a billet  11  pressed against a plug  22  mounted on a rod so as to obtain a pierced blank  12 . The rod is connected to a thrust block  26  designed to oppose the axial thrust which is produced on the rod during piercing. The plant also comprises a longitudinal mill  30  which is designed to roll the pierced blank  12  on a mandrel  32  so as to obtain a semi-finished tube  13 . The mandrel is connected to a retaining block  34  designed to impart to the mandrel a predetermined speed during rolling. In the plant according to the invention the thrust block  26  and the retaining block  34  are formed by a single thrust/retaining block  36.    
     The invention also relates to a method for rolling a seamless tube.

The present invention relates to a compact plant for rolling tubes, inparticular for rolling seamless tubes. The invention also relates to amethod for this rolling operation.

It is known to produce seamless metal tubes by means of successiveplastic deformation of a billet. During a first step the billet isheated in a furnace to about 1280° C. Then the billet is fed to a rotarypiercer comprising a pair of skew-axis rolls and a plug mounted on arod. The rolls axially push the billet against the plug, so that thebillet is pierced along its longitudinal axis. The rod is connected to athrust block designed to oppose the axial thrust which is produced onthe rod during piercing.

A semi-finished product (called pierced blank or hollow body), with athick wall and length 1.5 to 4 times the one of the starting billet, isthus obtained. Then the pierced blank is displaced laterally and theplug and the rod are extracted from it in order to be able to insert amandrel for the subsequent rolling operation. The mandrel has thefunction of opposing internally the radial thrusts which will be appliedduring rolling in order to thin the tube wall.

In order to complete the operations for lateral displacement of thepierced blank, removal of the plug and insertion of the mandrel, acertain amount of time is required. During this time period the piercedblank is exposed to the action of the air which causes in particular areduction in the temperature of the pierced blank. Moreover, the actionof the atmospheric oxygen results in the rapid oxidation of thesurfaces. For this reason a treatment (i.e. deoxidation treatment) usingdeoxidising materials, such as borax or the like, is usually carriedout.

As already mentioned above, a mandrel for the subsequent rollingoperation is then inserted inside the pierced blank. In some knownplants, the mandrel is connected to a retaining block designed to imparta predetermined speed to it during rolling.

The pierced blank is then passed through a multi-stand rolling mill withlongitudinal material flow (which will be referred to below as“longitudinal mill”) able to gradually thin the wall through a suitablereduction of the outer diameter, thus increasing the length of thefinished product. This rolling mill, as is known, comprises a pluralityof rolling units. Each unit comprises a stand on which rolls withprofiled grooves are mounted. The set of connected profiles of thegrooves of the rolls defines the outer profile of the tube released bythe rolling unit. Downstream of the longitudinal mill, the tube must befirst extracted from the mandrel. According to the prior art, thisoperation is performed by means of another rolling mill which generallyhas three or more stands, called an extracting mill. The extracting millperforms a small reduction in the outer diameter of the tube, with themain aim of allowing the mandrel to perform the stroke necessary forextraction from the semi-finished tube which has just been produced,also called “mother tube”.

Finally, the tube must be sized so as to define more precisely its outerdiameter. These functions may be performed by a corresponding number ofindependent machines, called an extracting mill, reducing mill andsizing mill, respectively. More often a single machine may insteadperform several functions and thus, for example, the extracting/reducingmill or the reducing/sizing mill is obtained. Then the tube must onlyundergo the inspection and/or finishing operations. The inspectionoperations may consist of pressure tests and checks which arenon-destructive, for example using ultrasound. The finishing operationsmay instead consist of cutting to size, cleaning, painting and marking.

The set of operations described above constitutes the core of the tubeproduction process; in fact at the end of these operations the maincharacteristics of the tube (i.e. the wall thickness and the outerdiameter) are defined. In a corresponding manner the assembly consistingof rotary piercer, longitudinal mill, extracting mill, reducing milland/or sizing mill defines the main part of the plant, also calledtechnological part. The entire of the layout of the production site, interms of civil engineering and structural work, must be designed on thebasis of the technological part.

On the other hand, the apparatus necessary for the inspection and/orfinishing operations may be usually arranged without significantconstraints around the main part of the plant.

It is known that the combination of transverse flow rolling (performedin the piercer) together with longitudinal flow rolling (performed inthe multi-stand rolling mill) ensures qualitative characteristics of theend product which are superior to that obtained with variousconfigurations, for example of the type using transverse double rolling(i.e. with a double piercer).

It is also known that the rolling in which the mandrel is retained andextracted on-line immediately downstream of the rolling mill ensuresquality of the finished product and reliability far greater than thoseof other known configurations, in which the mandrel is released and/orfree and in which extraction thereof is performed afterwards andoff-line.

It is also known that multi-stand longitudinal rolling is suitable forobtaining very high diameter/thickness ratios, where “diameter/thicknessratio” is understood as meaning the ratio between the outer diameter ofthe tube and the thickness of the tube wall. In particular, thediameter/thickness ratios obtained by means of multi-stand longitudinalrolling reach values of about 48-50, i.e. much higher than the ratioswhich may be obtained with other configurations of a different kind,such as that using transverse flow double rolling, where the ratios aregenerally less than 30-35.

The known plants of the type described above, although widelyestablished, are not entirely satisfactory.

Typically such plants are designed to achieve optimum tube production ona very large scale, for example in the region of 500,000 tonnes peryear. In view of the production on such a vast scale and in order toexploit fully the economies of scale, the plants of the known typeproduce so-called double-length semi-finished tubes, i.e. with lengthsof up to about 32 metres. Obviously all the dimensions of the plant mustbe defined on the basis of this final tube size. This size and otherparameters are such that, for example, the main building of theproduction site must have dimensions which exceed the standard marketdimensions. Such a building may for example have a main span which isnearly 45 metres and a length which measures more than 900 metres. Asthe person skilled in the art may clearly understand, a span of nearly45 metres requires the construction of an ad hoc beamwork and overheadcrane, since prefabricated parts with such dimensions are not available.

For these reasons also, the construction of the entire plant, includingcivil engineering and structural work, requires a huge initial outlay,such that an economic return may be guaranteed only if there isproduction on a vast scale as mentioned above.

Alongside this type of large-scale production plant there are alsoplants for production on a smaller scale (50,000 tonnes per year orless). These plants require a much smaller initial outlay and usuallyemploy technologies which are older and less efficient in terms ofprocess yield and process costs per tonne produced.

The aforementioned plants designed for production on a large scale arealso affected by other problems. For example, the transportation of thepierced blank from the piercer to the longitudinal mill may require from50 to 70 seconds. During this time the pierced blank tends to disperseheat into the environment, thus reaching the longitudinal mill in asubstantially colder condition, typically at a temperature of betweenabout 1050° C. and about 1150° C., depending on the form of the saidpierced blank. Obviously, the more the temperature of the pierced blankdrops, the more the forces which must be applied by the rolling millincrease in order to obtain the desired plastic deformation of thematerial. The value of the forces to be applied determines the size ofthe actuators and the motors of the rolling mill, as well as the powerconsumption of the latter during rolling.

As already mentioned, when the plug of the rotary piercer is extracted,the internal cavity of the pierced blank is struck by a flow of ambientair, the oxygen content of which inevitably results in the formation ofscale on the inner surface of the pierced blank. This effect must becounteracted because the presence of scale inside the tube during thesubsequent processing steps would give rise to the formation ofunacceptable defects. For this purpose, the pierced blank is treatedinternally with anti-oxidising agents, typically borax or substancessimilar thereto. As the person skilled in the art may easily understand,the industrial use of these substances creates major problems for thesafety of the environment and of the workers owing to the toxicity ofthe substances used.

Moreover, the need for anti-oxidising treatment negatively affects thetransfer time of the pierced blank between rotary piercer andlongitudinal mill and the overall costs of the production procedure.

The object of the present invention is therefore that of overcoming atleast partially the drawbacks mentioned above with reference to theprior art.

In particular, a task of the present invention is to provide a plant forrolling seamless tubes which may be more compact.

Moreover, a task of the present invention is to provide a plant for thehot rolling of seamless tubes which is sustainable from theenvironmental point of view due to the reduction, during the normalproduction process, of the consumption of electricity and gas, of theemission levels and of the pollutants.

Moreover, a task of the present invention is to provide a compact plantfor rolling seamless tubes which allows the negative effects of exposureto air of the pierced blank to be avoided or at least reduced, thusreducing the formation of scale and increasing the yield of the plant.

Finally, a task of the present invention is to provide a compact plantfor the rolling of seamless tubes which requires a smaller initialoutlay and which allows the operating costs to be kept low.

The object and tasks indicated above are achieved by a plant inaccordance with that claimed in Claim 1 and by a method according toClaim 12.

The characteristic features and further advantages of the invention willemerge from the description, hereinbelow, of a number of examples ofembodiment, provided by way of a non-limiting example, with reference tothe accompanying drawings in which:

FIG. 1 shows a plan view diagram of a complete rolling plant accordingto the prior art;

FIG. 2 shows, on the same scale as FIG. 1, a plan view diagram of acomplete rolling plant according to the invention;

FIG. 3 shows the technological part of a plant according to the priorart, indicated by III in FIG. 1;

FIG. 4 shows, on the same scale as FIG. 3, the technological part of aplant according to the invention, indicated by IV in FIG. 2;

FIG. 5 shows a plan view of a plant according to the invention similarto that of FIG. 4;

FIG. 6 shows a detailed view of the plant part according to theinvention indicated by VI in FIG. 5;

FIG. 7 shows a possible variant of the plant shown in FIG. 6;

FIG. 8 shows another possible variant of the plant according to FIG. 6;

FIG. 9 shows a mandrel used in some plants according to the invention;

FIG. 10 shows a rod used in some plants according to the invention; and

FIG. 11 shows a type of mandrel/rod assembly used in some plantsaccording to the invention; and

FIG. 12 shows another type of mandrel/rod assembly used in some plantsaccording to the invention.

The plant for rolling a seamless tube according to the invention isdenoted in its entirety by 10. It comprises:

-   -   a rotary piercer 20 comprising two skew-axis rolling rolls and a        plug 22 mounted on a rod 24, the rotary piercer 20 defining a        rolling axis X_(PR) and a rolling sense and being designed to:        -   receive at its input a solid billet 11,        -   pierce completely the billet 11 against the plug 22 so as to            obtain a pierced blank 12, and        -   release the pierced blank 12 engaged on the rod 24 of the            plug 22;    -   a longitudinal mill 30 comprising a plurality of rolling        stations 32 with adjustable motor-driven rolls, the longitudinal        mill 30 defining a rolling axis X_(LL) and a rolling sense and        being designed to:        -   receive at its input the pierced blank 12 released by the            rotary piercer 20;        -   roll the pierced blank 12 on a mandrel 32 so as to obtain a            semi-finished tube 13, and        -   release the semi-finished tube 13.

Moreover the rod 24 of the plug 22 of the rotary piercer 20 is connectedto a thrust block 26 designed to oppose the axial thrust which isproduced on the rod 24 during piercing and designed to impart to the rod24 and to the plug 22 the movements necessary before and after piercing;and

the mandrel 32 of the longitudinal mill 30 is connected to a retainingblock 34 designed to impart to the mandrel 32 a predetermined speedduring rolling and designed to impart to the mandrel 32 the movementsnecessary before and after rolling.

Moreover, in the compact plant 10 according to the invention, the rotarypiercer 20 and the longitudinal mill 30 have the same rolling sense; andthe thrust block 26 and the retaining block 34 are obtained by means ofa single thrust/retaining block 36.

With reference to the rolling plant 10, the rotary piercer 20 defines arolling axis X_(PR) and the longitudinal mill 30 defines a rolling axisX_(LL). Since the rolling axes X_(PR) and X_(LL) are parallel to eachother, “axial” or “longitudinal” will refer to the direction of astraight line parallel to the axes. Similarly “transverse” will refer tothe direction of a straight line not parallel to the axes. Finally“lateral” or “vertical” will refer to the direction of a half lineperpendicular to one of the two rolling axes.

Moreover the processing steps define specifically a rolling sense.Expressions such as “before”, “preceding”, “upstream” or the like referto positions which along the rolling axis are relatively close to thestart of the plant. On the other hand, expressions such as “after”,“following”, “downstream” or the like refer to positions which along therolling axis are relatively close to the end of the plant. In the lightof the above, it should also be considered that the upstream end of theworkpiece is also referred to as “tail”, while the downstream end isalso referred to as “head”.

As mentioned above, the rotary piercer 20 and the longitudinal mill 30have the same rolling sense, namely the same orientation along the axialdirection. In other words, with particular reference to FIGS. 5 to 8,during the rolling steps the workpiece (which in the various stagesassumes the name of “billet” 11, “pierced blank “12 and “semi-finishedtube”13) moves from right to left without reversing the sense of itsaxial movement: during piercing the workpiece moves from right to leftalong the axis X_(PR) of the rotary piercer 20; then, duringlongitudinal rolling, it moves from right to left along the axis X_(LL)of the longitudinal mill 30. In accordance with some embodiments (see inparticular FIGS. 5 and 6) the pierced blank 12 output from the piercer20 also undergoes a lateral displacement so as to pass from the axisX_(PR) to the axis X_(LL). In accordance with other embodiments (see inparticular FIGS. 7 and 8) the pierced blank 12 must instead not undergoany lateral displacement because the axes X_(PR) and X_(LL) coincide. Inaccordance with some embodiments of the invention it is also possiblefor the workpiece (for example the pierced blank 12) to be displacedslightly in the sense opposite to the rolling sense, but this occursonly during the movement which is required between the two successiverolling steps.

As already mentioned above, in the plant 10 according to the invention,the thrust block 26 and the retaining block 34 are formed by a singlethrust/retaining block 36. In connection with the present description,the expressions “thrust block 26” and “retaining block 34” simply definetwo different logic functions of the single block actually present inthe plant 10, referred to as “thrust/retaining block 36”. In accordancewith some embodiments of the invention, the thrust/retaining block 36comprises a first gripping unit 260 for the rod 24 and a second grippingunit 340 for the mandrel 32 and both the gripping units are mounted on asame thrust bearing unit 362.

More particularly, therefore, the thrust/retaining block 36 comprisesthe first gripping unit 260 to which the rod 24 is connected. The firstgripping unit 260, which is mounted on the thrust bearing block 362, isabove all designed to grip the rod 24 and transfer the axial thrusts tothe thrust bearing unit 362; particularly important is the transfer ofthe axial thrust which is produced on the rod 24 during piercing. Infact, during piercing, the rolls push the billet 11 against the plug 22and the thrust is transmitted via the rod 24 to the first gripping unit260 and then to the thrust bearing unit 362. The thrust bearing unit 362is in turn designed to transmit this thrust to the ground. Moreover, thethrust/retaining block 36 is also designed to impart to the rod 24 andthe plug 22 the movements which are necessary before and after rolling.The thrust bearing unit 362 is in fact designed to move the firstgripping unit 260, and along with it the rod 24 and the plug 22. Thesemovements occur in the axial direction and designed to move the plug 22from a rest position to a working position and vice versa.

In accordance with some embodiments, the first gripping unit 260 is alsodesigned to impart to the rod 24 and to the plug 22 a pre-rotation aboutthe rolling axis X_(PR). The possibility of obtaining the pre-rotationof the plug 22 is particularly advantageous in some cases because itallows, in a known manner, piercing of the billets to be facilitated.

In turn, the thrust/retaining block 36 to which the mandrel 32 of thelongitudinal mill 30 is connected comprises advantageously a secondgripping unit 340 mounted on the thrust bearing unit 362. Thethrust/retaining block 36 is above all designed to grip the mandrel 32and to impart to it a predetermined speed during rolling. In fact,during rolling, the rolls of the longitudinal mill push the piercedblank 12 in an axial direction and this transmits the thrust to themandrel 32. The mandrel 32 is firmly gripped by the second gripping unit340 and the axial thrust which is thus generated is transmitted to thethrust bearing unit 362. The thrust bearing unit 362 is in turn designedto transmit this thrust to the ground. If the mandrel 32 were notretained it would slide along the rolling mill 30 together with thepierced blank 12. On the other hand, in this type of plant, the mandrel32 is braked by the thrust/retaining block 36. The thrust/retainingblock 36 is also designed to impart to the mandrel 32 the movementsneeded before and after rolling. These movements occur in an axialdirection and are intended to move the mandrel 32 from a rest positionto a working position and to extract the mandrel 32 from thesemi-finished tube 13 released by the longitudinal mill 30.

By means of the solution of combining the thrust block 26 and theretaining block 34 in a single thrust/retaining block 36, it is possibleto simplify the rolling plant 10 along its portion situated between therotary piercer 20 and the longitudinal mill 30. In particular thesynergy which is obtained arises from the fact that both the thrustblock 26 and the retaining block 34 must be designed to generate forcesand displacements in the axial direction. In the thrust/retaining block36, therefore, by means of a single system it is possible to manage boththe rod 24 of the rotary piercer 20, i.e. move it and withstand itsthrust, and the mandrel 32 of the longitudinal mill 30, i.e. move andretain it. By way of example, the system for moving the thrust bearingunit 362 may consist of a carriage, which is able to move by means of apinion which meshes along a rack, or of a carriage which is able to movealong guides by means of cables or chains.

A favourable variant of the embodiment of the present invention is onein which the thrust/retaining block 36, in addition to a single thrustbearing unit 362, also comprises a single gripping unit 360. The singlegripping unit 360 is designed to grip both the rod 24 and the mandrel 32and transmit the corresponding axial thrusts to the thrust bearing unit362. This variation of embodiment improves substantially the mechanicalsynergy between the components to the advantage of costs, space andcycle times. Moreover, In accordance with some embodiments, the singlegripping unit 360 is also designed to impart to the rod 24 and to theplug 22 a pre-rotation about the rolling axis X_(PR).

Moreover, by replacing the thrust block 26 and the retaining block 34with a single thrust/retaining block 36 it is possible to obtain agreater compactness of the plant 10 in the longitudinal direction.

In accordance with some embodiments of the plant 10 according to theinvention, the rod 24 at the output of the rotary piercer 20 is axiallyaligned with the mandrel 32 and brought directly into contact with theend thereof. The pierced blank 12 may therefore be slid directly fromthe rod 24 to the mandrel 32 so that it can be conveyed away forlongitudinal rolling.

In accordance with some embodiments of the plant 10 according to theinvention, advantageously the rolling axes X_(PR) and X_(LL) coincide.This means that the pierced blank 12 output from the rotary piercer 20may be fed directly to the longitudinal mill 30, without any need todisplace it laterally. By means of a simple axial movement the piercedblank 12, still engaged on the rod 24 of the rotary piercer 20, isbrought into the vicinity of the mandrel 32 of the longitudinal mill 30in a simple and rapid manner This axial movement may be performedpreferably by means of suitable known devices which, by way of anon-exclusive example, may be motor-driven rolls of the pressure typecommonly known as “pinch rolls”, of the chain type with pins, or similartype.

In accordance with some embodiments of the plant 10 according to theinvention, the rod 24 of the rotary piercer 20 is structurally connectedto the mandrel 32 of the longitudinal mill 30. This solution allows themovement of the tooling, i.e. two tools, to be simplified since duringthe operational steps they are structurally connected to each other. Inthis way it is also possible to achieve perfect alignment of the twotools, this allowing the pierced blank 12 to be displaced easily,without the possibility of unexpected difficulties arising.

Particularly advantageous is the embodiment in which the mandrel/rodassembly 324 is used together with the single gripping unit 360 of thethrust/retaining block 36. This solution in fact is particularlycompact.

In particular, the embodiments of the plant 10 in which the rod 24 isstructurally connected to the mandrel 32 may advantageously use twosolutions. In accordance with a first solution, the rod 24 and themandrel 32 are made as one piece, while in accordance with a secondsolution the rod 24 and the mandrel 32 are connected togetherreleasably. Both the solutions have specific advantages and positivefeatures and the choice of one as opposed to the other may be madedepending on the specific needs.

With regard to the mandrel 32 and the rod 24 or the single tool referredto below as “mandrel/rod assembly 324” the following must also beconsidered: the rolling plant 10 according to the invention ispreferably designed to roll single-length tubes, namely semi-finishedtubes with a length of about 15 metres. Based on this measurement allthe main measurements of the plant 10 are defined, as the person skilledin the art may easily understand from the attached FIG. 5. Consideringthis final measurement of the tube and the elongation ratios of thevarious processing steps, it is possible to define a length of about 8metres for the rod 24 and about 15 metres for the mandrel 32.

In view of this it can be easily understood how also, in the case whereit is decided to use the mandrel/rod assembly 324, the management ofsuch a tool does not pose any difficulties. In fact, the distancebetween the rotary piercer 20 and the longitudinal mill 30 is greaterthan the overall length of the aforementioned mandrel/rod assembly 324and therefore allows easy movement, including lateral movement, of thelatter, for example for change-over after rolling or evacuation in theevent of an emergency.

Moreover, this tool may have an overall length of less than 25 metres,therefore consistent with the lengths of the mandrels normally used inconventional plants which produce double-length tubes.

Moreover, the mandrel/rod assembly 324 represents a more economicalsolution than the conventional mandrels which are comparable to it interms of length and weight. The latter in fact must have a treatedsurface (for example chrome-plated or with a similar finish) called“noble part” 320 extending over practically their entire externalsurface. The mandrel/rod assembly 324, on the other hand, must have atreated surface only on the outer surface of the portion which acts as amandrel 32, while the portion which acts as a rod 24 does not requireany treatment of this kind (see also FIGS. 11 and 12).

Moreover, it is thus possible, using suitable systems known per se,called “terns” (not shown in the figures), to constrain the part of themandrel/rod assembly 324 which during piercing is stressed bycompressive loads, so as to prevent buckling, i.e. instability due toaxial loading. At the same time, the terns do not intervene on the noblepart 320 of the mandrel/rod assembly 324 which is intended forlongitudinal rolling, the surface of this part having been previouslytreated with lubricant (generally graphite-based) for subsequentlongitudinal rolling. Since the terns do not come into contact with thisnoble part 320 of the mandrel/rod assembly 324, the lubricantdistributed over it remains in good condition until the moment of therolling operation. This allows on the one hand a good quality of thefinished tube to be obtained and on the other hand the noble part 320 ofthe mandrel to be preserved, increasing consequently its working life.

As already mentioned with regard to operation of the plant 10, duringthe piercing performed by the rotary piercer 20, the rod 24 restsaxially on the thrust/retaining block 36 intended to transfer the axialthrust exerted by the rolls on the billet 11 and then transmitted fromthe latter to the plug 22 of the rotary piercer 20. At the end ofpiercing, the two gripping units 260 and 340 (or in certain cases thesingle gripping unit 360) are removed to allow the movement of thepierced blank 12. Once the pierced blank 12 is engaged on the mandrel32, the latter is connected to the second gripping unit 340 or to thesingle gripping unit 360. The thrust/retaining block 36 is intended toapply a tractional force on the mandrel 32 so as to impart to the lattera predetermined axial speed during rolling in the longitudinal mill 30.

If the rolling axes X_(PR) and X_(LL) coincide, the movement of thepierced blank 12 is reduced to a simple displacement in the axialdirection. In the light of the above comments, the person skilled in theart may certainly understand how the simple axial displacement of thepierced blank 12 reduces to a minimum its movement and, along with it,its transfer time. More particularly, the transfer time of the piercedblank 12 from the outlet of the rotary piercer 20 to the inlet of thelongitudinal mill 30 is thus limited to only about 15 seconds, comparedto the about 50-70 seconds which are needed in the known plants. Duringthis short time period the pierced blank 12 disperses much less heatinto the environment, thus reaching the longitudinal mill 30 in asubstantially hotter condition, typically at a temperature of betweenabout 1210° C. and about 1240° C., depending on the form of the saidpierced blank 12.

Moreover, the fact that the pierced blank 12 is displaced directly fromthe rod 24 to the mandrel 32 (or even more simply is displaced along themandrel/rod assembly 324) prevents its internal cavity from being filledwith ambient air and from being subject to oxidation. In this way it isavoided having to perform known anti-oxidising treatments, which aretypically borax-based.

As regards the rotary piercer 20, it may be a piercer with barrel rollsor a piercer with conical rolls. Both the solutions, which are notdescribed here because they are well-known to the person skilled in theart, have specific advantages and positive features and the choice ofone as opposed to the other may be made depending on the specific needs.

As already mentioned above, the rotary piercer 20 is designed to receiveat its inlet a solid billet 11 and to pierce it completely so as toobtain a pierced blank 12. In other words, the rotary piercer 20 formsinside the billet 11 a through-hole so that the pierced blank 12 assumesthe form of a short thick tube, completely pierced from one side to theother.

The rotary piercer 20 is a transverse rolling machine in which, namely,the axes of rotation of the rolling rolls are skew. “Skew” refers to thefact that the rolling axes are not parallel to each other and do not liein the same plane.

The longitudinal mill 30 may also be designed with forms which differslightly. In general this rolling mill has advantageously the rollsmovable radially in order to adjust the position thereof and thereforethe force applied during rolling, resulting in the formation of thesemi-finished tube 13 from the pierced blank 12.

The rolling mill 30 is a longitudinal rolling mill in which, that is,the axes of the rolling rolls of each stand lie in a same plane, saidplane being orthogonal to the rolling axis.

Moreover, the longitudinal mill 30 is of the type with motor-drivenrolls, namely the movement of the pierced blank 12 along the rollingaxis X_(LL) is imparted by the rotation of the said rolls. This solutionoffers in a known manner advantages compared to the other solutions inwhich the movement is imparted in a different manner. For example, inthe solution referred to as a “thrust bench”, the movement along therolling axis X_(LL) is imparted by pushing the mandrel by means of aunit situated upstream of the rolling mill. This requires necessarily amandrel substantially longer than the semi-finished tube which isreleased from the rolling mill. Moreover, in order to obtain the thruston the mandrel, it is required to provide upstream of the rolling mill aspecial thrusting unit which is somewhat complex and in turn bulky.Moreover, this solution requires that the head of the pierced blankshould have a form which is able to withstand the thrust of the mandrel.Owing to this form, the head of the pierced blank must then be cut fromthe tube, thereby resulting in a reduction in terms of efficiency of theplant.

As mentioned above, the longitudinal mill 30 is designed to performrolling on a mandrel, in particular on a retained mandrel. In fact, theretaining block 34 imparts to the mandrel 32 a predetermined speedduring rolling. In particular, the axial speed imparted to the mandrel32 is greater than 0 mm/s and less than 4000 mm/s. The speed imparted tothe mandrel 32 during rolling therefore distinguishes the plantaccording to the invention from various other plants of the known type.Plants are in fact known where:

-   -   the mandrel is locked, i.e. has an axial speed of 0 m/s;    -   the mandrel is completely free (or floating), i.e. has an axial        speed which is not known a priori; and    -   the mandrel is pushed by an upstream unit, i.e. has an axial        speed equal to that of the head of the pierced blank.

In accordance with some embodiments of the plant 10, the longitudinalmill 30 is of the type comprising two rolls for each rolling station.This type of longitudinal mill is particularly compact, reliable andlow-cost. According to this type of rolling mill, for example, since thetwo rolls of each station are operated by the same motor, it issufficient to use a number of motors equal to the number of rollingstations contained therein. This obviously results in advantages interms of initial cost, operating consumption, complexity (which islower) and overall reliability.

According to other embodiments of the plant 10, the longitudinal mill 30is of the type comprising three rolls for each rolling station. Thistype of longitudinal mill is particularly precise and suitable for theproduction of high-quality tubes. According to this type of rollingmill, for example, the three rolls of each station manage to transmitthe deformation force in a particularly regular manner onto the outerprofile of the tube. This clearly results in advantages in terms ofquality of the end product. Also as regards the longitudinal mill 30,both the solutions, which are not described here because they arewell-known to the person skilled in the art, have specific advantagesand positive features and the choice of one as opposed to the other maybe made depending on the specific needs.

According to some embodiments, the plant 10 according to the inventionalso comprises an operating fork 40 for extracting the mandrel 32 fromthe semi-finished tube 13. This operating fork is positioned immediatelydownstream of the longitudinal mill 30 and is designed to replace theextractor mill present in conventional plants. At the end of the rollingof the semi-finished tube 13, the latter exits the longitudinal mill 30and remains engaged for a short portion on the mandrel 32. In fact, thelongitudinal mill imparts to the tube a significant axial speed, whilethe mandrel 32 contained inside it is retained by the retaining block 34upstream. This results in relative sliding of tube 13 and mandrel 32,such that they are almost entirely extracted. Once the tube has left thelongitudinal mill 30, the operating fork 40 retains it in position andthe mandrel 32 may be extracted backwards by applying a sufficient axialtractional force via the thrust/retaining block 36.

As the person skilled in the art may easily understand, the replacementof a complex and bulky machine such as the extractor mill with a simpleand compact device such as the operating fork 40 results in significantadvantages in terms of compactness, simplicity and overall reliability.

After extraction from the semi-finished tube 13, the mandrel 32 mustundergo a series of treatments so that it may be available for useagain. The treatments consist mainly of a suitable cooling and surfacetreatment with lubricant, which is typically graphite-based. Specialstations intended for these treatments are positioned alongside therolling plant 10.

Downstream of the operating fork 40, the plant 10 according to theinvention may also comprise a stretch-reducing mill 50, whereby saidmill may be advantageously preceded by a small induction furnace 60. Thestretch-reducing mill 50 is designed to receive at its inlet thesemi-finished tube 13 and to release the finished tube 14, at leastdepending on its main characteristics (i.e. thickness of the wall andthe outer diameter). Downstream of the stretch-reducing mill 50, thefinished tube 14 may if necessary undergo other secondary operations,such as inspection and/or finishing operations.

The presence of the induction furnace 60 immediately upstream of thestretch-reducing mill 50 may be advantageous if in that position of theplant 10 there is a reduction in the temperature of the semi-finishedtube 13. With regard to the above comment, this reduction in temperatureshould be avoided or kept at a negligible level, but in some particularcases the use of the induction furnace 60 could however be advantageous.For example, in the case of high-alloy steel tubes, the temperaturerequired for rolling in the stretch-reducing mill 50 could beadvantageously raised in order to reduce the rolling stresses.Furthermore, raising of the temperature during this step could beadvantageous in the case of tubes with a particularly thin wall whichare more exposed to cooling during the—albeit—small movements. It shouldbe noted here how the induction furnace 60 constitutes a particularlycompact and simple solution for dealing with the need to raise thetemperature of semi-finished tube 13. This furnace in fact hasparticularly small dimensions since it is formed simply by a shorttunnel inside which the semi-finished tube 13 is axially displaced.Special circuits create induction currents in the metal mass of the tubeand these currents determine heating thereof.

In accordance with other embodiments of the plant 10, thestretch-reducing mill 50 may be replaced by a sizing mill. The choice ofone as opposed to the other solution may be made on the basis of thespecific needs such as the size of the final tube, without adverselyaffecting the advantages of the invention.

The rolling plant 10 according to the invention also comprises a seriesof escape ways which prove to be particularly advantageous in emergencyconditions, namely when there is a malfunction in the rolling process.

A first escape way 71 is situated alongside the path for feeding thebillet 11 to the rotary piercer 20. This escape way 71 may be used todiscard a billet 11 which is ready for rolling, in the event of aproblem of any kind occurring along the rolling line. In this case, infact, it is not possible to feed the billet to the piercer 20 becausethe downstream plant may not receive other parts before the currentproblem is resolved.

In the event of a problem occurring during piercing, it may happen thatthe billet 11 may still be extracted from the plug 22 through forwardsdisplacement of the latter. In this condition, therefore, the onlypartially pierced billet 11 may be extracted from the rod 24 and theplug 22 downstream of the rotary piercer 20. In order to extract theplug 22 from the partially pierced billet 11 a first emergency fork maybe advantageously used, said fork being designed to keep the billet 11in position, while the rod 24 and the plug 22 are extracted forwards.The forwards extraction of the plug 22 and the rod 24 is performed bymeans of the thrust/retaining block 36. The only partially piercedbillet 11 may then be removed laterally or vertically, for example bymeans of a crane, while the rod 24 and the plug 22 may be removed bymeans of the normal lateral exit system, used after rolling.

Similarly, should the plug 22 remain blocked inside the billet 11, alsothe two parts together may be fed forwards, downstream of the piercer20, and discharged laterally. One possible lateral exit is the normalexit path of the mandrel 32, in particular if the rod 24 and the mandrel32 are structurally connected together. Another possible escape way isvertical and is made possible by a crane which may raise the assemblyconsisting of the billet 11 and the rod 24 inserted inside it, thusremoving them from the rolling line. The further operations needed toremove the plug 22 from the billet 11 may at this point be dulyperformed off-line, in the manner known per se.

A second escape way 72 and a third escape way 73 are arranged in thevicinity of the outlet for the semi-finished tube 13 from thelongitudinal mill 30. In the case of a problem occurring duringlongitudinal rolling, it may happen that the pierced blank 12 may stillbe extracted from the mandrel 32 by displacing the latter backwards. Inthis condition, therefore, the only partially rolled pierced blank 12may be moved forwards downstream of the longitudinal mill 30 and fromhere discharged laterally into the second escape way 72.

In order to extract the mandrel 32 from the partially rolled piercedblank 12, advantageously it is possible to use, depending on thecircumstances, the operating fork 40 or a second emergency fork 42situated immediately upstream of the longitudinal mill 30 and designedto keep the pierced blank 12 in position, while the mandrel 32 isextracted backwards. The extraction, backwards, of the mandrel 32 isperformed by means of the thrust/retaining block 36.

Should, on the other hand, the mandrel 32 remain blocked inside thepierced blank 12, the two parts may be fed further forwards downstreamof the longitudinal mill 30, along the third escape way 73 which formsessentially an extension of the outlet of the longitudinal mill 30. Inthis case, the rolling rolls of the stands of the longitudinal mill 30are positioned outside of the occupied volume by means of the adjustmentdevices which are normally used during rolling, so as to allow thepierced blank 12 and the mandrel 32 inserted inside it to pass through.

The further operations needed to remove the mandrel 32 from the piercedblank 12 may at this point be duly performed off-line, in the mannerknown per se.

In accordance with a further aspect thereof, the invention also relatesto a method for rolling a seamless tube. The method according to theinvention comprises the steps of:

-   -   providing a solid billet 11;    -   providing a rotary piercer 20 which defines a rolling axis        X_(PR) and a rolling sense and comprises two skew-axis rolling        rolls and a plug 22 mounted on a rod 24;    -   providing a longitudinal mill 30 which defines a rolling axis        X_(LL) and a rolling sense and comprises a plurality of rolling        stations with adjustable motor-driven rolls, the longitudinal        mill 30 being designed to roll a pierced blank 12 on a mandrel        32;    -   feeding the solid billet 11 to the rotary piercer 20;    -   piercing completely the billet 11 against the plug 22 so as to        obtain a pierced blank 12;    -   opposing the axial thrust which is produced on the rod 24 during        piercing;    -   releasing a pierced blank 12 engaged on the rod 24 of the plug        22;    -   displacing the pierced blank 12 from the rod 24 of the plug 22        to the mandrel 32 of the longitudinal mill 30;    -   feeding the pierced blank 12 engaged on the mandrel 32 to the        longitudinal mill 30;    -   rolling the pierced blank 12 on the mandrel 32 so as to obtain a        semi-finished tube 13;    -   retaining the mandrel 32 so as to impart to it a predetermined        speed during rolling; and    -   releasing the semi-finished tube 13.

Moreover, in the method according to the invention, the rotary piercer20 and the longitudinal mill 30 have the same rolling sense and thesteps of opposing the axial thrust on the rod 24 and retaining themandrel 32 are performed by means of a single thrust/retaining block 36.

According to some modes of implementing the method according to theinvention, the rotary piercer 20 defines a rolling axis X_(PR), thelongitudinal mill 30 defines a rolling axis X_(LL) and the rolling axesX_(PR) and X_(LL) coincide.

As will be clear to the person skilled in the art, the rolling plant andmethod according to the invention overcome at least partially thedrawbacks identified with reference to the prior art.

In particular, the person skilled in the art will clearly recognize theadvantages arising from the characteristic features of the inventionwhich enable a particularly compact rolling plant 10 to be obtained. Inthis connection, for example, the main building of the production sitemay have much smaller dimensions. As regards the technological areas(shown in FIGS. 3 and 4) the width of the plant according to theinvention is 15% to 30% smaller than that of a conventional plant, whilethe length is 60% smaller. As the person skilled in the art may fullyunderstand, such a reduction in space results in major savings in termsof initial investment and management costs. For example a prefabricatedbeamwork and overhead crane may be used for the span of about 30 metrespresent in a plant according to the invention, thus allowingconsiderable savings compared to the special reinforced overhead cranesneeded for spans for example of 45 metres or more.

Considering the overall dimensions of the lines (shown in FIGS. 1 and2), the area occupied by the plant according to the invention is reducedby 50% compared to a conventional plant.

Moreover, it will be clear to the person skilled in the art how, as aresult of the invention, in particular in some of its embodiments, asubstantial reduction in the movement of the pierced blank 12 betweenthe outlet of the rotary piercer 20 and the inlet of the longitudinalmill 30 is achieved. The substantial difference in temperature whicharises, as a result, at the inlet of the longitudinal mill 30 hasalready been mentioned, this temperature having values in the region of1210-1240° C. instead of 1050-1150° C. The effect of such a temperaturemay be fully appreciated when one considers that, following the testscarried out, the Applicant has established that the plant 10 accordingto the invention, all other conditions being equal, may use a nominalrolling power per stand which is even 35% lower than that of a plant ofthe known type.

Similarly, as a result of the plant 10 according to the invention, thepower consumption during normal operation may be reduced. As analternative to a reduced power consumption, rolling of steels with ahigher binder content which otherwise could not be processed is possiblewith the plant according to the invention.

Further effects of the reduction in the transfer time of the piercedblank 12 are associated with the management of the mandrels 32. Firstly,the higher temperature of the pierced blank 12 results in smallerstresses on the external surface of the mandrel 32 and therefore lesswear. Moreover, the smaller transfer time of the pierced blank 12 givesrise to a shorter overall cycle time and this leads to the possibilityof reducing the number of mandrels 32 which must be simultaneouslypresent in the plant 10 in order to ensure operation thereof. Thisnumber of mandrels, which in the case of conventional plants is equal toabout 6 or 7, in the plant according to the invention is reduced to 3.Obviously it is necessary to consider that the noble part 320 of thesingle mandrel 32 used in the plant 10 according to the invention isabout half that of a conventional mandrel 32.

All these aspects mentioned above result in significant savingsassociated with the overall management of the mandrels 32. Also inconnection with the mandrels, a smaller environmental impact due to thesmaller overall area of the surfaces to be treated superficially—forexample a smaller area of the noble part 320 of the mandrel to betreated by means of chrome-plating, multiplied by a smaller overallnumber of mandrels 32—is obtained.

Another particularly advantageous aspect is the possibility ofeliminating completely the borax-based deoxidation treatments. In thisway, not only the costs associated with the provision and use of thetreatment station are avoided, but also the use of toxic substances iseliminated, with consequent reduced environmental impact. A more rapidtransfer of the pierced blank 12 is also obtained, this beingparticularly advantageous for heat-related reasons in the case of apierced blank 12 which is particularly thin and therefore subject tomore rapid cooling.

The person skilled in the art, in order to satisfy specificrequirements, may make modifications to the embodiments of the plant andthe method for rolling seamless tubes according to the invention and/orreplace the parts described with equivalent parts, without therebydeparting from the scope of the accompanying claims.

1. Compact plant (10) for rolling a seamless tube, comprising: a rotarypiercer (20) comprising two skew-axis rolling rolls and a plug (22)mounted on a rod (24), the rotary piercer defining a rolling axis X_(PR)and a rolling sense and being designed to: receive at its input a solidbillet (11), pierce completely the billet (11) against the plug (22),and release a pierced blank (12) engaged on the rod (24) of the plug(22); a longitudinal mill (30) comprising a plurality of rollingstations with adjustable motor-driven rolls, the longitudinal milldefining a rolling axis X_(LL) and a rolling sense and being designedto: receive at its input the pierced blank (12) released by the rotarypiercer; roll the pierced blank (12) on a mandrel (32), and release asemi-finished tube (13); wherein: the rod (24) of the plug (22) of therotary piercer (20) is connected to a thrust block (26) designed tooppose the axial thrust which is produced on the rod (24) duringpiercing and designed to impart to the rod (24) and to the plug (22) themovements necessary before and after piercing; and the mandrel (32) ofthe longitudinal mill (30) is connected to a retaining block (34)designed to impart to the mandrel (32) a predetermined speed duringrolling and designed to impart to the mandrel (32) the movementsnecessary before and after rolling; and wherein the rotary piercer (20)and the longitudinal mill (30) have the same rolling sense; and thethrust block (26) and the retaining block (34) are formed by a singlethrust/retaining block (36).
 2. The plant (10) according to claim 1,wherein the thrust/retaining block (36) comprises a first gripping unit(260) designed to grip the rod (24)5 and a second gripping unit (340)designed to grip the mandrel (32), both the gripping units being mountedon a single thrust bearing unit (362).
 3. The plant (10) according toclaim 2, wherein the first gripping unit (260) and the second grippingunit (340) of the thrust/retaining block (36) form a single grippingunit (360).
 4. The plant (10) according to claim 1, wherein the rollingaxes X_(PR) and X_(LL) coincide.
 5. The plant (10) according to claim 1,wherein the rod (24) of the plug (22) of the rotary piercer (20) isstructurally connected to the mandrel (32) of the longitudinal mill(30).
 6. The plant (10) according to claim 5, wherein the rod (24) ofthe plug (22) of the rotary piercer (20) and the mandrel (32) of thelongitudinal mill (30) are formed as one piece.
 7. The plant (10)according to claim 5, wherein the rod (24) of the plug (22) of therotary piercer (20) and the mandrel (32) of the longitudinal mill (30)are releasably connected together.
 8. The plant (10) according to claim1, wherein the rotary piercer (20) is chosen from the group comprisingpiercers with barrel rolls and piercers with conical rolls.
 9. The plant(10) according to claim 1, wherein the longitudinal mill (30) is chosenfrom the group comprising rolling mills with two-roll rolling stationsand rolling mills with three-roll rolling stations.
 10. The plant (10)according to claim 1, further comprising an operating fork (40) forextracting the mandrel (32) from the semi-finished tube (13).
 11. MethodA method for rolling a seamless tube, comprising the steps of: providinga solid billet (11); providing a rotary piercer (20) which defines arolling axis X_(PR) and a rolling sense and comprises two skew-axisrolling rolls and a plug (22) mounted on a rod (24); providing alongitudinal mill (30) which defines a rolling axis X_(LL) and a rollingsense and comprises a plurality of rolling stations with adjustablemotor-driven rolls, the longitudinal mill (30) being designed to roll apierced blank (12) on a mandrel (32); feeding the solid billet (11) tothe rotary piercer (20); piercing completely the billet (11) against theplug (22) so as to obtain a pierced blank 12, and opposing the axialthrust which is produced on the rod (24) during piercing; releasing thepierced blank (12) engaged on the rod (24) of the plug (22); displacingthe pierced blank (12) from the rod (24) of the plug (22) to the mandrel(32) of the longitudinal mill (30); feeding the pierced blank (12)engaged on the mandrel (32) to the longitudinal mill (30); rolling thepierced blank (12) on the mandrel (32) so as to obtain a semi-finishedtube (13); retaining the mandrel (32) so as to impart to it apredetermined speed during rolling; and releasing the semi-finished tube(13). wherein the rotary piercer (20) and the longitudinal mill (30)have the same rolling sense and the steps of opposing the axial thruston the rod (24) and retaining the mandrel (32) are performed by means ofa single thrust/retaining block (36).